The present invention relates to a communication apparatus wirelessly coupled to an intended receiver by the spread spectrum method.
Since the Federal Communications Commission (FCC) has opened Industrial Scientific and Medical (ISM) bands to the public as a communication tool using the spread spectrum method, numbers of new products employing the spread spectrum method have been put in various market segments, such as cordless telephones, of communication industry. The spread spectrum method features its inherent resistance to interference and enhancing privacy of communication. FCC has approved a transmission by the spread spectrum method with a larger power than a conventional analog communication method. Therefore, the spread spectrum method has an advantage of enlarging a communicable distance.
Products employing this method are, in general, designed based on digital communication method, and they are not only more expensive but also have much complicated structures than those designed based on analog method. An analog based spread spectrum communication apparatus employing a simple and inexpensive circuit on top of the conventional apparatus is thus available in the market, and it can perform spread-spectrum and de-spread spectrum. This apparatus is illustrated in FIGS. 13A and 13B.
An information signal fed into transmitter 400 shown in FIG. 13A is frequency-modulated (FM) by FM modulator 401 in the same manner as the conventional analog communication, thereby producing narrow-band FM signal 501. In spreader 402, signal 501 is modulated into spread-spectrum modulated signal 502 by spreading-code 504 produced by pseudo noise (PN) code generator 404. Then signal 502 is converted and amplified into a high frequency band by radio frequency (RF) transmitter 403, whereby signal 503 is radiated into the air from transmitting antenna 405.
Antenna 411 of receiver 410 illustrated in FIG. 13B receives this radio wave signal, and RF receiver 412 amplifies this signal. The resultant signal tapped off from RF receiver 412, i.e. RF spread-modulated signal 512 and de-spreading code 515 produced by PN code generator 415 are electrically multiplied together in de-spreader 413. In this multiplication, sync circuit 416 controls timings of de-spreading code 515 thereby synchronizing code 515 with signal 512. As a result, original narrow band FM signal 513 is obtainable as an output signal from de-spreader 413. This FM signal 513 is converted and filtered by intermediate frequency (IF) section 414 into IF signal 514, which is demodulated to the original information signal by FM demodulator 418.
Sync circuit 416 performs the following correlative operation thereby realizing de-spreading. De-spreading code 515 used in the de-spreading is identical to spreading code 504 used in the spreading, and is tapped off at the same speed as that of code 504. A signal level of the signal tapped off from IF section 414 is supplied as output voltage 614 of a Received Signal Strength Indicator (RSSI). If the output voltage 614 is monitored, it is understood that when a phase of de-spreading code 515 is synchronized with that of RF spreading modulated signal 512, i.e. the correlation value takes a maximum value, the RSSI voltage reaches to the maximum level. In other words, the de-spread is realized when the maximum RSSI voltage is acquired and maintained by sliding the phase of de-spreading code 515. In actual, as shown in FIG. 13B, sync circuit 416 transmits phase-adjusting-signal 516 to PN code generator 415 thereby adjusting the phase of its output signal 515, and then RSSI voltage 614 is digitized by A/D converter 417 thereby obtaining RSSI data 517. A peak value of data 517 is acquired and maintained.
In the event of sync-loss during the communication, it is impossible to restore the sync when a sync-holding-mode is kept going, the sync circuit, therefore, should move immediately to a sync-tracking-mode from the sync-holding-mode, and try to acquire the sync. This is an outline of the performance.
In the case of digital communication, an xe2x80x9cout of syncxe2x80x9d can be detected with ease by monitoring an error-rate of the information demodulated finally in the receiver; however, in the case of analog communication, this method is difficult to use, and another method as follows is thus employed. Recognizing-signals such as data or tones dedicated to recognition only are transmitted during the communication, and the receiver monitors these signals to detect the xe2x80x9cout of syncxe2x80x9d. These recognizing-signals are independent of the information to be transmitted.
The analog-based spread spectrum communication apparatus discussed above is less expensive as well as simpler in circuit structure and operation than the digital-based one. Since the FM demodulator has inherent excellent receiving sensitivity, a maximum communicative distance can be extended. Further, a voice is free from being coded at transmission, a delay in the voice due to coding process never occurs. As such, the analog-based apparatus have several advantages over the digital-based one; however, it also has the following problems:
(1) When the receiver receives an intense signal, the de-spreader does not operate in a normal way due to saturation of the RSSI, i.e. the receiver has a narrow dynamic range.
FIGS. 14A and 14B are graphs illustrating influences of the saturation of a conventional RSSI to a detection of correlation. As points B in the Figs. illustrate, signal-to-noise ratio (SNR) at detecting correlation by the RSSI in the sync circuit deteriorates with regard to an input of which peak level exceeds a linearly-operating-region of RSSI. This status encourages sync-errors to occur. On the other hand, points A in the Figs. illustrate a case when the peak level of the input is not beyond the linearly-operating-region of RSSI. The RSSI mounted to general purpose ICs for IF has a dynamic range of ca. 60 dB in general, which is often short of the level required by various communication apparatuses, although the required levels depend on applications of the apparatuses.
(2) If the xe2x80x9cout of syncxe2x80x9d occurs due to fading or cross-talk, the sync-error rate cannot be monitored and thus it is difficult to detect the xe2x80x9cout of syncxe2x80x9d.
Although the method discussed previously is available, i.e. recognizing-signals such as data or tones dedicated to recognition only are always transmitted together with the information during the communication, and the receiver monitors these signals, this method requires a circuit separating the signals from the information, thereby complicating the circuit structure and operation as well as increasing the cost.
The conventional spread-spectrum-communication apparatus has thus two major problems as discussed above.
The present invention addresses the problems discussed above and provides a spread-spectrum-communication apparatus which employs a simple and inexpensive circuit, thereby settling an insufficient dynamic range of a receiver as well as stabilizing an operation against an intense signal. The apparatus of the present invention also detects an xe2x80x9cout of syncxe2x80x9d with reliability, thereby moving the operation immediately into a sync-tracking-mode.
A spread spectrum communication apparatus of the present invention comprises a transmitter and receiver.
The transmitter comprises the following elements:
an FM modulator for modulating an information signal in analog mode;
a spreader for converting an output signal from the FM modulator into a spread-spectrum signal;
an RF transmitter for converting the spreading signal into a high-frequency signal;
a transmitting antenna for radiating an output signal from the RF transmitter into the air.
The receiver comprises the following elements:
a receiving antenna for receiving the high frequency signal radiated from the transmitter;
an RF receiver for amplifying the received high frequency signal;
an RSSI for detecting an intensity of the signal;
a de-spreader for restoring the spread-spectrum signal into a narrow band FM modulating signal;
an FM demodulator for obtaining the original information signal from the modulated signal.
The apparatus further comprises a limiter circuit on a signal input side of the de-spreader so that stable de-spreading against an intense signal fed into the receiver can be expected. The apparatus also detects with reliability an xe2x80x9cout of syncxe2x80x9d during a communication thereby moving the operation immediately to sync-tracking mode.